I'm not a stepmum, but I think it was nosey, too. However, I see you know what she's paying for her upcoming holiday. Is it the norm in the family to discuss the cost of each other's vacations, etc? If so, she might not have thought she was doing anything out of the way, even though, imo, it was.
About her saying your vacation was "expensive." Was it intended as a criticism, do you think, or just an observation? Since, apparently, she spends a lot on holidays herself, she might not think there's anything wrong with yours being "expensive." Or she might think you spent a lot for a B & B, NOT that it's any of her business,
Or perhaps your dh has criticized what she spends on holidays, and this was her way of giving him a taste of his own medicine? Maybe not, just asking.
Am I right in thinking that you didn't raise sd and that's why you turned the question over to dh? If so, then, imo, you were very wise. I just wish he had said, "That's private." But again, perhaps they're used to sharing this kind of information? You might want to discuss this with him.